A combative, energetic performance from Biden — replete with sharp jabs at Ryan and regular outbursts of dismissive laughter — was the key ingredient of the night and seemed likely to drive the discussion in the aftermath.

To Democrats, Biden delivered exactly the kind of impassioned, scrappy performance they were desperate to see after President Obama’s widely criticized passivity in his initial clash with Romney last week.

“If Obama had been this strong, the election would be over now,” liberal talk show host Bill Maher wrote on Twitter.

But conservatives, on social media and elsewhere, suggested that the vice president appeared too “hot” and even rude toward a congressman 27 years his junior.

Biden came out firing from the outset, and in characteristic fashion. He dismissed as “a bunch of malarkey” Ryan’s first answer, which had tried to frame the September attack that killed four Americans in Benghazi, Libya, as part of a broader “unraveling” of Obama’s foreign policy.

There was much more to come in a similar light. When Ryan inveighed against the economic stimulus package passed in the early days of Obama’s presidency, Biden said he had received letters from Ryan seeking funds for Wisconsin.

At another point, after Ryan invoked the name of President John F. Kennedy, whom the Republican said had increased growth while lowering tax rates, Biden shot back: “Now you’re Jack Kennedy?”

But Biden’s body language was as striking as any words spoken by either man. Though he often wore a broad smile, he appeared disdainful of Ryan.

In being so demonstrative, Biden ran the risk of drawing comparisons with a previous Democratic vice president, Al Gore, whose sighing during his first 2000 presidential clash with then-Gov. George W. Bush was widely perceived to be self-damaging.

Biden’s “happy warrior” approach may well have been more successful, however. That was certainly the message from a CBS News snap poll in the immediate aftermath, in which 50 percent of a sampling of undecided voters said Biden won the debate as against 31 percent for Ryan. A CNN poll, however, found that Ryan bested Biden by 4 points, 48 percent to 44.

To the relief of Democrats, Biden committed none of the gaffes for which he has become infamous during his campaign trail appearances. He attacked Romney for his “47 percent remarks”, presented himself as a champion for (and product of) the middle class, and took issue with almost all of Ryan’s claims.

He also held his own in an exchange over abortion, which had been prefaced by a request from moderator Martha Raddatz for both men to speak in personal terms.

On the other hand, Ryan made a fluent case for the Republican ticket. He assailed Obama’s domestic and foreign policies as ineffectual while permitting little daylight to be seen between his own positions and those of Romney. He wrapped his arguments in a broader critique: that voters had given Obama a chance in 2008, and that he had let them down.

“It’s not working. It’s failed to create the jobs we need. This is not what a real recovery looks like,” he said near the conclusion of the debate.

Nor did Ryan hesitate to throw some pointed jibes of his own. Defending Romney’s “47 per cent” remarks, he said, “the vice president very well knows that the words sometimes don’t come out of your mouth the right way.”

Elsewhere, Ryan sought to turn Biden’s passion against him. “I know you’re under a lot of duress to make up lost ground,” he told him at one point.

But for all the passion displayed by both men, Thursday’s debate might not buck the historical trend: Vice presidential encounters tend to have little overall effect on the race.

Clashes between running mates often have produced memorable moments, perhaps the most striking being Sen. Lloyd Bentsen’s (D-Texas) devastating putdown of Sen. Dan Quayle (R-Ind.) as “no Jack Kennedy”.

But most pollsters find no evidence that any of the prior eight vice presidential debates of the TV era have had a meaningful impact on an election’s outcome. (The ticket that included Bentsen as potential vice president for Massachusetts Gov. Michael Dukakis, who was ultimately trounced by George H.W. Bush and Quayle, for example.)

Wildly disparate results from some recent polls have also added to the uncertainty that now hangs over the state of the race for the White House.

A Tampa Bay Times poll released just hours before Biden and Ryan took the stage had Romney up by 7 points among likely voters in Florida.

Yet, earlier in the day, an NBC News/Wall Street Journal poll showed Obama leading by a single point in the Sunshine State — exactly the same margin as a poll conducted just before the first presidential debate.

Overall, there is no doubt that there has been considerable tightening in the polls since that first Oct. 3. clash. New York Times polling expert Nate Silver estimates that Romney’s chances of winning the Nov. 6 election have more than doubled since then — though he continues to name Obama is the favorite.

Democrats will hope that Biden’s muscular performance in Danville, Ky., has stanched the bleeding, and given them a real chance to regain the momentum in the race.

Republicans, meanwhile, will be hoping that the simple lack of an obvious, unarguable victory for either side on Thursday will allow the tide to continue shift in their direction.

Among outside observers, opinion was also ambivalent.

“Biden repeatedly laughed at inappropriate moments. He looked a little too angry for his own good at certain moments — and he won the debate,” said Brad Phillips, a media consultant and the author of a widely read blog on media training.

“Congressman Ryan — who was more consistent in tone throughout the debate — too often appeared rehearsed, flat, and junior,” Phillips added.

“It was Biden Unleashed versus Ryan Unfazed,” Tobe Berkovitz, a Boston University professor who specializes in political communication told The Hill. “This served the needs of both campaigns. Obama desperately needed the base to be reinforced and Biden threw chunks of red meat to liberals throughout blue America.

“Ryan delivered a calm and polished performance. He was up to the task of fighting off Biden’s attack-fest.”

But, if neither man won, Biden has at least delivered a jolt to Democratic morale. After the panic of the last week, many Obama supporters will settle for that.